The present invention relates to a directly imageable waterless planographic printing plate precursor which can be directly processed by a laser beam.
A generally termed direct imaging type process of producing an offset printing plate directly from an original without using a process film has begun to be employed in the fields of general offset printing and flexographic printing, as well as in the short run printing field, due to such features as simplicity requiring no high skill, speediness in obtaining a printing plate in a short time, and rationality of selectability from a variety of systems in accordance with quality and cost.
Especially recently, in response to the rapid progress of output systems such as prepress systems, image setters, laser printers, etc., new types of various planographic printing plate materials are being developed.
The processing methods for the planographic printing plates include methods of irradiating with a laser beam, methods of drawing with a thermal head, methods of locally applying a voltage by a pin electrode, methods of forming an ink repellent layer or an ink acceptable layer by ink jet, etc. Among them, the methods using a laser beam are superior to the other methods in resolution and processing speed, and are diverse.
The laser beam-adapted printing plates can be grouped into two types: a photon mode type that utilizes photoreactions, and a heat mode type that causes heat reactions through light-to-heat conversion. The heat mode type has an advantage of allowing handling in a daylight room. With fast progress of semiconductor lasers for use as the light source, usefulness of the heat mode type methods is lately reviewed.
With regard to the heat mode type waterless planographic printing plates, the following proposals have been made.
For example, U.S. Pat. Nos. 5,339,737 and 5,353,705, and EP 0580393, etc. disclose thermal-breakdown type directly imageable waterless planographic printing plates that use a laser beam as a light source.
A heat sensitive layer in these planographic printing plate precursors contains mainly carbon black as a laser beam absorbing compound, and nitrocellulose as a thermally decomposable compound. The laser beam absorbed by carbon black is converted into heat energy, and the heat destroys the heat sensitive layer. Finally, the destroyed area is removed by a development process, whereby a silicone rubber layer on the surface simultaneously falls off, thus forming an image area.
A problem of the thermal-breakdown type printing plate is that since the heat sensitive layer is destroyed to form an image, the ditch cells of the image area become deep so that the ink acceptability deteriorates at very small half tone dots. In addition, the ink mileage is low. Furthermore, since the heat sensitive layer is provided with a crosslinked structure in order to facilitate thermal breakdown, the printing plate is poor in printing durability. Still another problem is that this type of printing plate has low sensitivity, and therefore requires a high-intensity laser beam in order to destroy the heat sensitive layer.
As measures for removing these drawbacks, Japanese Patent Application Laid-Open Publication Nos. 2000-330266 and HEI 11-268436 disclose waterless planographic printing plate precursors which have a heat sensitive layer and an ink repellent layer on a substrate, and allow formation of an image by reducing the adhesion between the heat sensitive layer and the ink repellent layer through conversion of the laser beam into heat. These patent applications also disclose provision of a heat insulating layer between the substrate and the heat sensitive layer.
Another requirement is the proofing by reading a printing plate with measuring apparatus, for example, by a method in which the reflection density of a printing plate is measured with a densitometer. However, the aforementioned known printing plates do not allow measurement of the dot area ratio on the printing plates through the use of a densitometer or the like.